Communications systems and particularly wireless communications systems require transmitters in one form or another. Transmitters and particularly higher power transmitters, specifically power amplifiers (PAs), are one of the more expensive and more highly stressed, from a power perspective, components of a communications system. By virtue of the PAs relatively intimate coupling to an antenna system they are subject to abuse due, for example, to antenna irregularities, such as mismatches and weather or lightning events. For these reasons transmitters or power amplifiers are subject to or often experience relatively higher failure rates than some system components. These higher power transmitters are typically found in base stations. Base stations, such as those found in cellular or like communications system are a key link in providing satisfactory service for hundreds and possibly thousands of customers.
Therefore carriers or service providers or network operators cannot afford to have a transmitter failure and will go to great lengths to avoid failures or at least service outages when there is a failure. Hence most base station suppliers utilize some form of redundancy for power amplifiers within the base stations they provide to the marketplace. More recently some manufacturers have been using radio frequency Fourier Transform Matrices (FTMs) to provide this redundancy. FTMs combine phase shifted versions of input signals to provide multiple output signals which are then amplified and passed through another radio frequency FTM to decompose the amplified signals into amplified versions of the original input signals. Thus if a PA fails, the remaining PAs will continue to amplify the input signals and provide amplified signals thus avoiding service outages. This may be more economical than having full redundancy for each power amplifier. However, there is still a problem.
Many communications systems today require linear PAs because they rely on complex modulation schemes that specify amplitude and phase variations in the transmitted radio signals. Even systems that do not per se require linear PAs, but use FTMs, will find amplitude variations in the combined phase shifted signals that are to be amplified and thus need linear PAs. Linear PAs are very difficult and expensive to construct. Limiting the range of output, thus input, signals over which the amplifier must demonstrate a degree of linearity can control or contain costs and difficulties. In conventional amplifier systems there are known techniques for processing the input signals to assist with these goals, however there are no known techniques that work when FTMs are employed. What is needed is a method and apparatus for processing base band signals using transformations thereof to improve performance of transmitters, specifically multi-channel transmitters using FTMs.